During the early months of 1998, 2002, 2004 & 2006, many corals on the Great Barrier Reef, Australia and many other reefs in the world changed their colouration from brown to a brilliant white, purple or green. This ‘whitening’ of the coral is called coral bleaching and is due to a loss of the symbiotic dinoflagellates (algae) living within the tissue of the coral. In a healthy reef, the algae supply the coral with energy (sugars and amino acids) and in turn receive products essential to their survival (ammonia and phosphate).
Without the brown zooxanthellae present in healthy coral, bleached coral will display the bright white calcium carbonate skeleton and fluorescing pigments.
Many stressful environmental conditions can lead to bleaching, however, elevated water temperatures due to global warming have been found to be the major cause of the massive bleaching events observed in recent years. As the sea temperatures cool during winter, corals that have not starved may overcome a bleaching event and recover their symbionts.
However, even if they survive, their reproductive capacity is reduced, leading to long-term damage to reef systems. Sea temperatures are predicted to continue to rise, and thus, bleaching is expected to occur more and more frequently, leading to the death of large areas of coral reefs worldwide within a few decades!
Other major threats to corals reefs are ocean acidification, overfishing, coral disease, invasive species, and both natural and human disturbances. Read more about this in “Coral Reefs and Climate Change: the guide for education and awaress”.